1. Field of the Invention
The present invention relates to a control system in a rear-wheel-drive vehicle, a method of determining whether or not a wheelie has occurred in the rear-wheel-drive vehicle and a method of suppressing driving power generated in a driving source in the vehicle.
2. Description of the Related Art
In rear-wheel-drive vehicles such as motorcycles, a wheelie sometimes occurs when a high engine driving power is transmitted to a rear wheel. Japanese Laid-Open Patent Application Publication No. 2002-70709 discloses a control system in a motorcycle for controlling an engine to reduce an engine driving power upon the wheelie starting.
A conventional control system includes an acceleration sensor. When a wheelie is occurring, a vehicle body of the motorcycle is tilted such that a front wheel lifts off the ground. For this reason, a gravitational force component of acceleration, detected by the acceleration sensor, becomes larger when the wheelie is occurring than when the motorcycle is driving normally. The conventional control system utilizes this phenomenon and determines that the wheelie is occurring if the acceleration detected by the acceleration sensor is not less than (i.e., is greater than or equal to) a predetermined value. During a period when it is determined that the wheelie is occurring, the control system suppresses the engine driving power based on an output of the acceleration sensor. When the acceleration is less than the predetermined value, the control system determines that the wheelie is not occurring.
However, the output of the acceleration sensor contains noise. During driving, the vehicle body vibrates due to an engine operation and bumps and depressions on a road surface, which results in a fluctuation in the gravitational force component of the detected acceleration. The gravitational force component of the detected acceleration also fluctuates due to a slope of the road surface. For this reason, it is difficult to accurately detect whether or not the wheelie is occurring in the conventional control system using the acceleration sensor. As a result, it is difficult to properly suppress an engine driving power, as desired, in the conventional control system.